fix: resolve all ruff and ESLint lint errors

Backend (ruff):
- F821: Add TYPE_CHECKING imports for Vehicle, Contact, File in models
  (file.py, retouch.py, sale.py, vehicle.py)
- E741: Rename ambiguous variable  to /
  (copilot_service.py, price_compare_service.py, openrouter.py)
- F841: Remove unused variable  in sale_service.py

Frontend (ESLint):
- react/no-unescaped-entities: Escape quotes in ContractPreview.tsx
- @next/next/no-img-element: Replace <img> with <Image> from next/image
  (FileGallery.tsx, FileList.tsx, FilePreview.tsx, BeforeAfterSlider.tsx)

Tests: 392 backend passed, 112 frontend passed, next build successful

frontend/package.json

@@ -12,22 +12,24 @@
   },
   "dependencies": {
     "next": "14.2.5",
+    "next-intl": "3.17.0",
     "react": "18.3.1",
-    "react-dom": "18.3.1",
-    "next-intl": "3.17.0"
+    "react-dom": "18.3.1"
   },
   "devDependencies": {
-    "typescript": "5.5.4",
+    "@testing-library/jest-dom": "6.4.8",
+    "@testing-library/react": "16.0.1",
     "@types/node": "20.14.0",
     "@types/react": "18.3.3",
     "@types/react-dom": "18.3.0",
-    "tailwindcss": "3.4.7",
-    "postcss": "8.4.40",
+    "@vitejs/plugin-react": "4.3.1",
     "autoprefixer": "10.4.19",
-    "vitest": "2.0.5",
-    "@testing-library/react": "16.0.1",
-    "@testing-library/jest-dom": "6.4.8",
+    "eslint": "^8.57.1",
+    "eslint-config-next": "^14.2.35",
     "jsdom": "24.1.1",
-    "@vitejs/plugin-react": "4.3.1"
+    "postcss": "8.4.40",
+    "tailwindcss": "3.4.7",
+    "typescript": "5.5.4",
+    "vitest": "2.0.5"
   }
 }